Technical Abstract:
Over the past decade biobarriers, insoluble organic-rich zones introduced into the aquifer to stimulate microbial activity, have become an accepted method for removing contaminants from aquifer water. This study investigated the use of biobarriers in the unsaturated or vadose zone. In the 12-week-study water containing 20 mg/L nitrate-N was periodically applied to the surface of 10 by 145 cm columns and the water allowed to percolate through the sand filled columns. Two sets of columns, the positive control and the treatment groups, contained a 25 cm biobarrier formed by mixing 2,700 g of sand with 300 g of sawdust soaked in soybean oil (1:2). A third set of columns, the negative control, contained no biobarrier. The biobarriers in the positive control group were saturated throughout the study while the biobarriers in the other two groups were allowed to drain. Nitrate-N in the effluents from the treatment and positive control columns declined as the study progressed and averaged 0.4 ± 0.1 and 0.8 ± 0.1 mg/L N (respectively) during the last six weeks of the study, a greater than 95% reduction in nitrate-N. Moisture probes placed within the biobarriers show that the barriers removed nitrate even when the water content was in the 20 to 40% pore filled space range. In contrast, nitrates in the effluents from the negative control averaged 17.9 ± 0.4 mg/L N during the last half of the study. Regardless of treatment, only small amounts of ammonia, less than 0.5 mg/L-N, were present in the effluents. The results show that vadose zone biobarriers effectively removed nitrogen from groundwater as contaminated water percolated through the columns. Such barriers should provide a useful means of protecting aquifers from nitrate contamination.